Vertical-cavity, surface-emitting laser arrays offer many advantages for applications in optical communications, optical interconnects, optical neural networks and optical signal processing. Moreover, recent publications have demonstrated that such vertical-cavity, surface-emitting laser arrays can be made in a highly dense array and with low threshold currents.
Jewell et al disclosed such an advanced two-dimensional array of vertical-cavity, surface-emitting lasers in a technical article entitled "Low threshold electrically pumped vertical-cavity surface emitting microlasers" appearing in Electronics Letters, volume 25, 1989 at pages 1123-1124. Further details of the Jewell et al array are found in U.S. patent application, Ser. No. 07/380,996, filed July 17, 1989, now issued as U.S. Pat. No. 4,949,350, and incorporated herein by reference. Jewell et al grew a vertical epitaxial semiconductor structure consisting of upper and lower distributed Bragg reflectors separated by an optical distance equal to the lasing wavelength to form a vertical resonant cavity. An active quantum well layer was placed in the middle of the cavity. After this planar structure was grown, the array of lasers was defined by an etching process. Electrical current was passed vertically through a selected laser, causing it to emit light vertically.
There are numerous variations of this basic structure and its fabrication procedure appearing in the literature. However, all known references to arrays of vertical-cavity, surface-emitting lasers have attempted to fabricate arrays with a uniform lasing frequency, which follows from the initial uniform growth of the vertical structure prior to lateral definition. However, there are several applications which can advantageously use one or two dimensional arrays of lasers having separate laser frequencies. Examples of such systems are wavelength division multiplexing (WDM) optical communication systems and optical signal processors. For example, in a WDM system, each laser would be modulated by a data channel and would emit at a separate optical carrier frequency assigned to that channel. All optical outputs would be combined on one optical fiber for WDM transmission.
A multiple-wavelength, edge-emitting (horizontal-cavity) laser array has been disclosed by Nakao et al in a technical article entitled "1.55 .mu.m DFB laser array with .lambda./4-shifted first-order gratings fabricated by x-ray lithography", appearing in Electronics Letters, volume 25, 1989 at pages 148-149. The wavelength dependence was achieved by x-ray lithographic definition of cavity gratings of slightly different periods. Such a lithographic approach fails for a vertical-cavity structure in which all the reflectors are simultaneously deposited in a single growth sequence and the peak wavelength of their reflectance is determined by the layer thicknesses.
Tai et al have disclosed a type of vertical-cavity laser array in a technical article entitled "Use of implant isolation for fabrication of vertical cavity surface-emitting laser diodes" appearing in Electronics Letters, volume 25, 1989 at pages 1644-1645. They reported that their lasers had lasing wavelengths ranging from 863 to 854 nm. They ascribed the variation to nonuniform layer thickness across the wafer grown by their MBE (molecular beam epitaxy) method.
In MBE, the particle sources, called effusion cells, are located a few tens of centimeters from the substrates and located at a substantial angle off the normal of the substrate. It is well known that geometrical and other effects cause a non-uniform deposition on stationary substrates, producing varying layer thicknesses. Thus, the standard practice in MBE growth, particularly of devices having critical thicknesses, such as vertical-cavity lasers, is to rotate the substrate during MBE deposition so that all parts of the wafer are exposed to nearly the same distribution of angles of deposition. Nonetheless, as reported by Tai et al, layer uniformities do remain.